Cupcakes:
6 tbsp (3/4 stick) unsalted butter, cut into 4 pieces
1 1/4 cups semisweet chocolate chips (about 8 ounces)
3 ounces unsweetened chocolate, chopped
1/2 cup (packed) golden brown sugar
1/3 cup sugar
2 large eggs
1 tsp. vanilla extract
1/2 cup all-purpose flour
1/4 tsp. salt

Frosting:
1 cup powdered sugar
3/4 cup creamy peanut butter (do not use old-fashioned or freshly ground)
1/4 cup (1/2 stick) unsalted butter, room temperature
1/4 teaspoon vanilla extract
4 teaspoons (about) whipping cream (optional)